Ignition coil with resistor

[Synchro909]

I had an ignition coil fail on me recently and the only one I could find to replace it at short notice was supposed to have a resistor used with it. Fortunately, someone there knew how to connect it, Is it just connected in series with the coil? What is the point of it anyway? Is the coil actually made for a lower voltage and the resistor is left out of the circuit while the battery is cranking the motor and therefore only able to deliver a lower voltage but drops the voltage to the coil during normal running?
I've converted to 12 volts.

[Mike V. Florida]

If you have converted to 12 volts you need to coil and resistor combination. Either in the coil or external to it.

For those that are 6 volts, you need a coil designed for an external resistor.

There were ign systems that were designed to add and remove resistance during starting and running, but not the A.

http://en.wikipedia.org/wiki/Ignition_coil

[Synchro909]

Originally Posted by Mike V. Florida

If you have converted to 12 volts you need to coil and resistor combination. Either in the coil or external to it.

For those that are 6 volts, you need a coil designed for an external resistor.

There were ign systems that were designed to add and remove resistance during starting and running, but not the A.

http://en.wikipedia.org/wiki/Ignition_coil

Mike, Thanks for the reply.
I'm aware the A didn't have a resistor. The coil I have is clearly marked 12 Volts, yet it has a resistor. It is therefore clearly intended to run on less than 12 volts. It is not a 6 volt coil in a 12 volt system with a resistor to drop the voltage at the coil to 6 volts.

[Bob Bidonde]

When 12-volt ignition systems first came out about 1955, car makers continued to use the 6 to 8-volt coils they had with the 6-volt ignition systems, so a resistor was used to reduce the voltage. A three position ignition switch was used so when starting the car, the resistor was bypassed and the coil got 12 volts. When the key went into the run position, the resistor went back into the circuit so the coil ran with 6 to 8 volts. Some coils had the resistor built-in, and others had an external resistor.

High energy ignitions (HEI) systems resulted in another change, and coils in this type of system are truly 12-volt coils.

[Mike V. Florida]

With a coil that uses an external resistor you should see 1.2 to 1.4 ohms across the primary.
Coils with an internal resistor (for 12volt systems) should read about twice of that above, 3 ohms.

Coil test

[Patrick L.]

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A coil with an internal resistor is to reduce the voltage to the points. Points do not like 12 volts.

[marc hildebrant]

Originally Posted by Patrick L.

A coil with an internal resistor is to reduce the voltage to the points. Points do not like 12 volts.

You meant to say current as the voltage isn't a problem for the points.

As others have said, the use of a 6 volt coil running on 12 volts during the start of the engine helps to deliver a "hotter" spark during the startup of the engine.

Marc

[Joe K]

A coil with an internal resistor is to reduce the voltage to the points. Points do not like 12 volts.

Kettering (developer/refiner of the coil/points ignition) found that 4 amps is about the practical limit for current in a points type ignition. Thus resistance to limit current through the coil.

Discussion of the difference in resistance of a 6v and 12v coil at http://www.yesterdaystractors.com/cg...rtips&th=26558 - and the use of resistors to achieve a correct current.

Joe K

[Bruce Lancaster]

And...there are quite a few different values of resistance in the resistor, so when buying a coil match up the resistor at same time by the catalog rec or from OEM usage. And, of course, find out if it even uses one or has its resistance internally! Reistor application is not generic.
And there were also 6 V coils that used a resistor too, including early V8 Fords and some others. Don't buy a coil and a resistor separately without reference to what the coil requires.

[Tom Wesenberg]

Did your other coil fail because it was oil filled and mounted upside down?

[Synchro909]

Originally Posted by Tom Wesenberg

Did your other coil fail because it was oil filled and mounted upside down?

No, Tom. One morning while we were away on a rally, I couldn't start the car. It had been a cold night (by our standards - about 5C (40F)). It didn't take long to track down the culprit. Fortunately, another guy on the rally had a coil and resistor. Had to replace the head gasket on another car that morning too - not an A. Funny how you remember the glitches better than the rest of the rally.

[Willie Krash]

Just to add to the conversation the starter was tied in front of the resistor and powered the coil during the cranking cycle. If you're cranking the engine and the battery breaks down to 9.5-10V you get better firing voltages to the plugs while cranking. Every volt counts toward upping KV firing voltage
Chev used this early on in a "foot feed" starter. The switch used a resistor bypass.
You may notice starter solenoids are marked S and R

[dansluck]

Tom I live in rural Nevada and when you break down it is a long way to get parts. I always install a coil with primary facing up. Learned that as a kid. Good Day to ya. Dan

[Patrick L.]

I'm just trying to keep this simple. I'm smart enough to understand about current. Most folks like simple, especially me.
That said, points do not like anything over 6-8 volts.
Run 12 volts thru a set of points without a resistor/reducer stuck somewhere in front of them and see how long they last. Been through it.

[marc hildebrant]

Patrick L,

Just trying to help. The 12 volts isn't the problem with the points...rather the increased current via ohms law is what damages the points.

If you know this, then I'm just trying to keep the story accurate for those that are reading this thread and may not understand why 12 volts is a problem ?

After all, many cars were built that ran 12 volts and points.

Marc

[Patrick L.]

Originally Posted by marc hildebrant

Patrick L,

Just trying to help. The 12 volts isn't the problem with the points...rather the increased current via ohms law is what damages the points.

If you know this, then I'm just trying to keep the story accurate for those that are reading this thread and may not understand why 12 volts is a problem ?

After all, many cars were built that ran 12 volts and points.

Marc

Which one[s] ran 12 volts through the points ?

[marc hildebrant]

Patrick L.

My 66 Olds Cutlass used 12 volts and points....

Not sure though why the question as many cars had 12 volts and coil\points.

Marc

[Patrick L.]

Originally Posted by marc hildebrant

Patrick L.

My 66 Olds Cutlass used 12 volts and points....

Not sure though why the question as many cars had 12 volts and coil\points.

Marc

That didn't answer my question. Pretty much all cars in the 50s/60s were 12 volt and used points. But, none ran 12 volts through the points for more than a couple seconds, most didn't allow 12 volts through them at all.

[Afordman31]

Snyder’s installation instructions:
　

A-12000 COIL

This coil is imprinted that it is 12-volts, use with
external resistor. If you are running 6-volts, the coil is good for 6-volts as it is, ignore the 12-volt writing on the side. If you are running a 12-volt system, you will have to add an external ballast resistor to the
coil. This coil is only 12-volt if it is used with an external ballast resistor.

Afordman31